In 1956, a paper written by Vincent J. Frilette in the Journal of Physical Chemistry entitled "Preparation and Characterization of Bipolar Ion-Exchange Membranes", described a phenomenon known as "polarization", which occurred when a cation exchange membrane bonded intimately with an anion exchange membrane was placed in water or an aqueous electrolyte and a DC electrical field was applied with the cation exchange membrane surface facing the cathode .crclbar. pole. Frilette showed that under a low voltage applied across the system, hydrogen ions migrated toward the cathode and hydroxide ions migrated toward the anode. The only plausible explanation for this was polarization, or water splitting at a lower energy than electrolysis (gas generation), at the interfacial plane of the anion and cation exchange membrane. EQU 2H.sub.2 O.fwdarw.H.sub.3 O.sup.+ +OH.sup.-
If the polarity was reversed, sodium and chloride ions would pass from a slightly saline solution and accumulate at the bipolar membrane interface. Since the discovery of the "bipolar" membrane effect by Frilette, numerous bipolar membrane structures have been synthesized and patented. All of these membranes, without exception, polarize in a DC electric field and all split water, giving off protons toward the cathode and hydroxide ions toward the anode.
Several patents have been issued during the past three decades since Frilette published his discovery of the bipolar membrane effect. These have covered the single film bipolar membrane, and the double film bipolar membrane. Each patent has made claims of superior durability, superior electrical conductance, and superior performance. None however addressed membrane caustic stability or a water supply to the bipolar membrane interface which are objects of the present invention.
In the case of the single bipolar membrane, F. B. Leitz in U.S. Pat. No. 3,562,139 describes the preparation of a bipolar membrane out of a single plastic film. The membrane was prepared by immersion of a 3 mil polyethylene (PE) film in a bath of styrene and divinyl benzene, polymerizing the same to give a polystyrene-polydivinyl benzene crosslinked polymer within the interstices of the PE film and then partially sulfonating one side of the aromatized film to form a cation exchange membrane followed by chloromethylation and amination of the opposite face of the film to form a single film bipolar membrane. This membrane split water in an electric field, but at a low current density of 9.3 milliamperes per centimeter square at a current efficiency of 58-66% (production of acid per theoretical Faraday of electricity).
Oda et al U.S. Pat. No. 2,829,095 disclosed a process for preparing acid and base solutions by use of a two sheet bipolar membrane employed in a cell along with a single polar cation and anion exchange membrane. These single membranes were prepared by separately mixing finely divided cation and anion exchange resins with polyvinyl chloride, with the bipolar membrane being put together by adhering each type of the above cation and anion exchange membranes with an epoxy resin comprising polyethylene imine with epichlorohydrin and hardening the adhesive layer. Indeed, using 0.5M NaCl, acid and base were generated at 20.8 ma/cm.sup.2 with a voltage of 2.9 volts per cell group, with each cell group consisting of three membranes positioned between electrodes, i.e., a bipolar membrane (with the cation side facing the cathode), a single polar cation membrane and a single polar anion membrane respectively. This was indeed a patent demonstration of acid and base production from a sodium chloride feed using the bipolar membrane to split water into its acid and base components.
Dege and Liu, U.S. Pat. No. 4,024,043 describe another single film bipolar membrane claiming an ion selectivity above 80% in at least a 0.1 molar(M) solution of salt. The membrane synthesis is similar to that of Leitz, with claims made that membranes made had high performance data (for single film bipolar membranes) of 0.86 volts @ 109 ma/cm.sup.2 up to 1.14 volts @ 91 ma/cm.sup.2 between a 1N KOH and a 1N HCl solution. Current efficiencies were between 68 and 78% respectively.
Improvements in the preparation of single and double film bipolar membranes are further described by F. P. Chlanda, L. T. C. Lee, and K. J. Liu (U.S. Pat. Nos. 4,116,889 and 4,253,900) where double film bipolar membranes are prepared by fusing slurries of mixed fine cation and anion exchange resins in solvents between commercially available anion and cation exchange membranes. High acid and base current efficiencies were obtained using less than 10% solutions of HF and KOH at current densities of close to 100 ma/cm.sup.2.
In 1987 Liu and Lee described in U.S. Pat. No. 4,673,454 a process for preparing an improved bipolar membrane, again of a double laminate, having high permselectivity and low potential drop. Using 1N Na.sub.2 SO.sub.4 in the test cell, voltages of 1.2 volts to 2.0 volts at 100 amps/ft.sup.2 were obtained across these double laminated membranes during the preparation of acid and base.
While the above listed patents describe ways and means of preparing single and double film bipolar membranes, along with voltage drops obtained at different current densities using different salts and solution concentrations; no mention has been made of water accessibility to the bipolar membrane interfaces or the expected life of the anion exchange portion in terms of caustic stability. The preparation of novel bipolar membranes having one thin component layer for facilitating a constant water supply to the bipolar interface and proven caustic stability of the anion exchange membrane portion will be the principal objects of this invention.